Heretofore, SMC articles generally containing fiber reinforcement therein have been made by compression molding and optionally in-mold coating the article. Subsequently, the entire exterior surface of the article as well as the edges thereof has been primed or top coated with a liquid coating. However, due to volatiles and/or gases contained within the SMC article, during cure and/or baking of the liquid primer or top coat, the volatiles could migrate to the surface and form a bubble which subsequently could burst (i.e., pop) leaving a crater like abnormality on a desired smooth surface.
SMC articles generally have internal voids. Low profile additives in automotive type body panels may increase the tendency to create internal voids while minimizing the shrinkage of the article's exterior dimensions. Most molded thermoset parts have a resin rich layer on the surface which was in contact with the mold surface. This resin rich layer can partially block passage of gases from the voids to the surface. Any process which disrupts this resin rich layer can expose these internal voids of the article such as die-cutting, deflashing, routing, sanding, punching out holes, and drilling. Solvent treatment and handling of the article during manufacturing and transporting can also cause disruption of the resin rich layer. They can be distinguished easily from molded surfaces.
Surface defects in subsequent coatings caused by the escape of trapped gases are more prevalent on or near machined surfaces than more removed areas of the part. The term machined as used herein does not specifically require that a machine be used to create the surface. These machined surfaces are created after molding and/or in-mold coating has occurred. The voids or pores of the porous plastic can be from a size large enough to be visible to submicron sizes. The voids are believed to supply gases during heated cure cycles that can pass through partially cured coatings creating either holes or craters, i.e., paint pops.
In order to overcome these problems, U.S. Pat. No. 5,021,297 to Rhue et al. relates to a method of preheating the entire substrate at a temperature and time sufficient to degas the substrate with said temperature being above the cure temperature of the subsequently applied thermoset powder coating composition, immediately applying a thermoset powder coating composition to the entire exterior surface of the heated substrate while not allowing the substrate temperature to drop below the cure temperature of the powder coating composition, and curing the coated substrate at a temperature at or above the minimum cure temperature of the powder coating.
The above mentioned conditions prescribed by the Rhue process create several disadvantages in terms of surface appearance, quality and ease of operation. The application of a powder coating to the entire exterior surface of an SMC article tends to produce a non-smooth "orange peel" appearance, which can be visually unacceptable for many purposes. Perhaps more importantly, the restriction that the substrate temperature be held above the cure temperature of the powder coating during its application requires one or more of the following undesirable conditions:
1) The substrate must be preheated to temperatures substantially above the cure temperature of the powder coating, which can cause structural damage to the SMC article and to any adhesive used therein.
2) A powder coating material with low-temperature curing characteristics must be used, which greatly limits the varieties of powder coating materials which may be suitable.
3) The powder coating application booth must be heated to temperatures near or above the cure temperature of the powder coating, which can cause the powder to melt and plug in hoppers, delivery hoses, application guns, overspray reclamation equipment, etc.
4) The SMC article must be conveyed through the powder coating application booth (from preheat oven to cure oven) at high speed, which requires an excessive amount of powder coating application equipment (powder guns, pumps, hoppers, gun motion machines) in order to adequately coat the SMC article in the time allowed.
5) The length of the powder coating application booth (the distance from the preheat oven to the cure oven) must be minimized which in turn may restrict the recommended application space of one powder application gun to another.
Any or all of the above-defined conditions may be undesirable with regard to process feasibility and/or visual quality of the finished SMC article.